Microsoft has finally set a rollout date for Windows Mobile 10. According to Lumia's Facebook handle, Microsoft will start pushing out Windows Mobile 10 as a OTA update in December.
The response was to a user's query on when Microsoft was planning to update older Lumias. The Microsoft Lumia team replied, "Broad availability of Windows 10 on other devices, including mobile phones, will begin rolling out in December."
Back in July, Microsoft released a list of handsets that would get the Windows 10 Mobile update which includes Microsoft Lumia 430, Lumia 435, Lumia 532, Lumia 535, Lumia 540,Lumia 640, Lumia 640 XL, Lumia 735, Lumia 830, and Lumia 930. On its Support Page, Microsoft notes that devices with less than 8GB internal storage may not get the update.

It is our ambition to make an upgrade available for majority of Lumia smartphones running Windows Phone 8.1. Only phones running Lumia Denim (OS version 8.10.14219.341, or higher) can take advantage of the upgrade to Windows 10

The update schedule will obviously vary across regions and carriers but after the exclusive Microsoft event yesterday, things are about to get pretty interesting with Windows Mobile 10.